The present invention relates to a voltage-controlled oscillator for variably controlling the frequency of an output pulse in accordance with an input voltage.
A phase locked loop (PLL) is known to be used to generate a clock signal that is synchronized with a reference pulse signal. The PLL includes a phase comparator for comparing the clock signal generated by the PLL with the reference pulse signal, a low pass filter for generating DC voltage in accordance with the comparison result of the phase comparator, and a voltage-controlled oscillator (VCO) for generating the clock signal from a control voltage, or the DC voltage from the low pass filter. In the VCO, a signal based on the difference between the frequency of the clock signal and the frequency of the reference signal is applied as the control voltage to the VCO to perform feedback control. The VCO varies the frequency of the clock signal in accordance with the control voltage. Thus, the VCO generates clock signals synchronized with various frequency signals.
The VCO, which varies its gain (the varied amount of the frequency (control current) relative to the varied amount of a predetermined control voltage) to generate the clock signal, is optimal, for example, when generating the reference clock signal with a data recording device in accordance with the rotation of the disc medium, the rotation of which is controlled. That is, when the data recording device performs 2× speed recording, the gain of the VCO is varied in accordance with the rotation velocity to generate the reference clock signal that properly corresponds to the rotation of the disc medium regardless of changes in the rotation velocity of the disc medium.
The gain control of the VCO may be performed by adding a certain control voltage to the predetermined control voltage. However, in such a case, when providing a control gain for each control voltage to the VCO gain, complicated control must be performed. Thus, the gain control is complicated when varying the VCO gain by adding two control voltages related to the rotation speed of the disc medium, in for example, a data recording device provided with a 2×speed recording function.